Earlier this term, the school wrote to parents confirming that results in two exams had been quashed, although reading scores were allowed to stand.

It also launched an investigation into the incident and later told parents that it was "the sole action of the Year 6 class teacher who is no longer employed by the school".

"We have built on the currently robust procedures in order to mitigate against any such rogue incident in the future," the school said.

Susan Hall, Harrow’s Conservative leader, said: "It is grossly unfair that these children will not now receive some of their SAT results because of the irresponsible conduct of others.”

League tables for primaries across England show that 25 schools had their results annulled by the Standards and Testing Agency – part of the Department for Education – this year.

STA officials are brought in to investigate any suspicions raised over the “accuracy or correctness of a child’s results” in end-of-year tests.

It is believed that more results may be annulled later in the academic year because a number of investigations are still ongoing.

Officials insisted that the number of schools flouting exam rules represented a tiny proportion of the 16,000 primaries nationally.

Of the 25 cases raised this year, a number involved schools previously appearing close to the top of the tables, including Newton Farm.

In at least three cases, schools that had some results annulled went on to score 100 per cent in other – legal – tests this summer.

The penalty can be used for a number of breaches of exam rules, including:

• Opening test papers more than one hour in advance of the test without permission;

• Storing test materials incorrectly;

• Photocopying papers;

• Allowing teachers gain knowledge of the test and coach children before entering the exam hall;

• Failing to stop pupils cheating;

• Failing to cover wall displays in test rooms;

• Giving unsupervised rest breaks to children;

• Allowing test administrators to over-aid children during an exam.

A Department for Education spokesman said: “The number of schools where results have been amended or annulled due to maladministration this year is consistent with previous years and illustrates that the majority of schools administer the tests rigorously.

“Instances of maladministration that lead to the annulment or amendment of children’s test results are rare, but completely unacceptable.”